As part of ever stricter regulations relating to environmental protection, motor vehicle manufacturers are being compelled to design engines to be efficient and to have a low consumption. An automatic start-stop system is already available in almost all vehicles. Moreover, there is increasingly the possibility of switching off the internal combustion engine while coasting.
With such switching concepts for the internal combustion engine, it is hardly possible to use a mechanical vacuum pump for servo assistance of the braking force. There is therefore a demand for electric vacuum pumps for modern applications. These vacuum pumps often run dry since it is also no longer possible to supply the vacuum pump with oil in a manner dependent on a running internal combustion engine.
The absence of the engine noise of the internal combustion engine brings the operating noise of the vacuum pumps to the fore in terms of what is audible. In order to minimize these operating noises, various measures are implemented in order to reduce structure borne noise and airborne noise.
The publication WO 2011/134448 A2 discloses a vacuum pump having a pump housing, in which pump at least one pump housing part is formed by a sandwich-type sheet metal material comprising two sheet metal layers, between which is arranged a plastic layer, by means of which the sheet metal layers are vibrationally decoupled from one another. The vacuum pump can comprise a muffler, which is formed from the sandwich-type sheet metal material. Vacuum pumps having noise absorption caps are known from PCT/DE2013/100370 (not a prior publication). In addition, a multifunctional decoupling element is arranged between the pump housing surface and the noise absorption cap, said cap performing a sealing function and, in this prior art, also a valve function in addition to a noise decoupling function.
The decoupling element serves to decouple the noise absorption cap acoustically, particularly in respect of vibrations and/or structure borne noise occurring during the operation of the motor vehicle vacuum pump. By virtue of the acoustic decoupling of the noise absorption cap brought about by means of the decoupling element, unwanted development of noise during the operation of the motor vehicle vacuum pump can be considerably reduced. The decoupling element also forms a seal between the pump housing surface and the noise absorption cap with respect to the environment of the motor vehicle vacuum pump. A separate seal between the pump housing surface and the noise absorption cap can thus be omitted.
However, it is not possible, using the measures of damping the airborne noise by means of the noise absorption cap and damping the structure borne noise by means of the decoupling element, for all the structure borne noise to be suppressed and, specifically, for the noise peaks at the resonant frequencies to be reduced.
It is an object of the invention to further optimize a motor vehicle vacuum pump having a pump housing surface, on which a noise absorption cap defining a noise damping volume is mounted, in respect of unwanted development of noise during the operation of the motor vehicle vacuum pump.